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This invention relates to semiconductor diode lasers, more specifically, to solid-state semiconductor diode lasers, which have multilayered vertical optical cavities that comprise a substrate, an electrically pumped double-heterostructure light emitting diode active-region, and an optically pumped solid-state active medium; all being components typically disposed between two feedback providing contra-positioned light reflecting structures.
Solid-state semiconductor diode lasers have numerous applications in fields as varied as the automobile industry, medicine, scientific instrumentation, and telecommunications. Solid-state semiconductor diode lasers, specifically solid-state semiconductor diode lasers having a multilayered vertical optical cavity (i.e., vertical orientation that is perpendicular to the substrate of the semiconductor diode), which have become widely known, typically, as (VCSELs) xe2x80x9cVertical Cavity Surface Emitting Lasersxe2x80x9d.
Categorically, VCSEL light sources for use in gigabit-Ethernet network applications have been adopted in a remarkably short amount of time. Because VCSELs have a reduced threshold current, a circular output beam, and are inexpensive to manufacture at very high-volumes is what makes present day VCSELs particularly suitable for multimode optical fiber xe2x80x9cLocal-Area Networksxe2x80x9d (i.e., LANs). Selectively oxidized VCSELs contain an oxide aperture within its vertical cavity that produces strong electrical and optical confinement, enabling high electrical-to-optical conversion efficiency, but minimal modal discrimination that allows emission into multiple transverse optical-modes. Such multi-mode VCSELs make ideal local area network laser light sources.
However, because they are inexpensive to manufacture, new VCSELs that can singularly emit at the fundamental optical transverse mode are ever increasingly being sought-out for emerging applications including long and short-haul data-communications using single-mode optical fiber, barcode scanning, laser printing, optical read/write data-heads, and modulation spectroscopy. Achieving single-mode operation in selectively oxidized VCSELs is a challenging task, simply because the inherent index confinement within these high-performance lasers is very large. VCSELs have optical-cavity lengths on the order of one-wavelength and, therefore operate within a single longitudinal optical-mode.
Nevertheless, because of their relatively large cavity diameters (i.e., roughly xe2x80x9c5.0xe2x80x9d to xe2x80x9c20.0xe2x80x9d micrometers), these lasers will usually operate in multiple transverse optical-modes. Wherein, each transverse optical-mode will possess a unique wavelength and what is typically called a transverse spatial intensity profile (i.e., intensity pattern). For applications requiring small spot size or high spectral purity, lasing in a single transverse optical-mode, usually the lowest-order fundamental mode (i.e., TEM-00 mode) is desired.
In general, pure fundamental mode emission (i.e., a spatial intensity pattern seen as a simple circular shaped dot) within a selectively oxidized VCSEL can be attained by increasing the optical loss to higher-order transverse optical-modes relative to that of the previously mentioned fundamental lower-order transverse optical-mode. By selectively creating optical loss for any particular mode, we increase modal discrimination, which, consequently leads to a VCSELs operation in a single transverse optical-mode. Strategies for producing VCSELs that will operate in a single transverse optical-mode have recently been developed.
Furthermore, the previously mentioned strategies are based either on introducing loss that is relatively greater for higher-order optical-modes, thereby relatively increasing gain for the fundamental transverse optical-mode, or on directly creating greater gain for the fundamental transverse optical-mode. Increased modal loss for higher-order optical-modes has been demonstrated by three different techniques. The first approach to modal discrimination uses an etched-surface relief structure on the periphery of the top facet, which selectively reduces the reflectivity of the top mirror for the higher-order transverse optical-modes. The advantage of this technique is that the etched ring around the edge of the cavity in the top quarterwave mirror-stack assembly can be produced during the VCSEL""s fabrication using conventional dry etching, or be post processed on a completed VCSEL die using focused ion-beam etching. The disadvantages, however, of an etched-surface relief structure is that it requires careful alignment to the oxide aperture or it can increase the optical scattering loss for the fundamental transverse optical-mode, as manifested by the relatively low (i.e., less than 2-mW) single-mode output powers that have been already reported. Therefore, it would be more desirable to introduce mode-selective loss into a VCSEL""s epitaxial structure to avoid extra fabrication steps and problems with self-alignment and loss thereof.
Moreover, two such techniques are the use of tapered oxide apertures and extended optical cavities within VCSELs. The first approach pursued at Sadia National Laboratories (i.e., Albuquerque, N. Mex.) is typically called gain-guided design and it is predicated on designing the profile of the oxide aperture tip to preferentially increase loss for higher-order transverse optical-modes. For example, the aperture-tip profile can be produced by tailoring the composition of (AlGaAs) xe2x80x9cAluminum-Gallium-Arsenidexe2x80x9d layers, which are specifically oxidized to create an electro-optic aperture within the before mentioned VCSEL""s internal cavity. A VCSEL containing a tapered oxide structure whose tip is vertically and precisely positioned at the null of the longitudinal optical standing wave that occurs within the laser can produce greater than 3-mW of single transverse optical-mode output, and greater than 30-dB of side-mode suppression. Creating this kind of structure, however, requires a detailed understanding of the specifics regarding the oxidizing process and how it is implemented using specific materials, or tapered oxide structures improperly produced will cause additional loss rather than additional gain for the fundamental transverse optical-mode.
In addition, the second approach pursued at the University of Ulm (i.e., Ulm, Germany) is typically called index-guided design and it is predicated on designing a way to increase modal discrimination by extending the optical cavity length of VCSELs, thereby increasing the diffraction loss for the higher-order transverse optical-modes. Researchers at the University of Ulm (i.e., Ulm, Germany) have reported single fundamental optical-mode operation up to 5-mW using a VCSEL designed with a xe2x80x9c4xe2x80x9d micrometer thick cavity spacer inserted into its vertical optical cavity. The problem, however, is that by using even-longer cavity spacers we begin to introduce multiple longitudinal optical-modes (i.e., causing what is sometimes called spatial hole burning) into the laser""s system, however, single fundamental lower-order transverse optical-mode operation up to nearly 7-mW has been demonstrated using this approach. It is interesting to note that VCSELs containing multiple wavelength cavities do not appear to suffer any electrical penalty, however, careful design is required to balance the trade-offs between the modal selectivity over a VCSEL""s transverse and longitudinal optical-modes.
Finally, by manipulating the modal gain of a device rather than its loss can also produce single fundamental transverse optical-mode VCSELs. A technique to spatially aperture laser gain has been recently developed at Sadia National Laboratories. The essential aspect of a VCSEL designed using this approach is found in the VCSEL""s lithographically defined gain region, which is produced by an intermixing of the VCSEL""s quantum-well active-region at the lateral periphery of the VCSEL""s vertical cavity.
Typically, well-known epitaxial processes of material deposition, like MBE or MOCVD, are used to epitaxially deposit the various materials that comprise the multilayered structure a VCSEL device(s). The fabrication process for a VCSEL typically begins with an epitaxial deposition of a bottom DBR quarterwave mirror-stack assembly onto an optical or semiconductor substrate material. The process continues with an epitaxial deposition of a VCSEL""s active-region, which contains either a laser active medium comprising a single bulk semiconductor material or a single quantum well, or contains a laser active structure comprising a multiple quantum well or a multi-layered superlattice structure. The fabrication process for a VCSEL typically ends with an epitaxial deposition of a VCSEL""s top DBR quarterwave mirror-stack assembly. Moreover, the quantum wells typically used in VCSELs are homogenized by ion-implantation around masked regions that form the optical cavities of the VCSEL lasers. The resultant VCSELs have a central quantum-well active-region that preferentially provides gain for the fundamental transverse lower-order optical-mode.
Consequently, for this approach an output of more than 2-mW with a side-mode-suppression ratio greater than 40-dB was obtained for an output of fundamental transverse optical-mode emission. Although, this approach requires greater fabrication complexity, it is anticipated that higher performance can be reached with further refinement of fabrication process parameters. Because new and greater demands are being made by existing commercial and future applications of VCSEL technology, new types of single-mode VCSELs are currently under development at numerous laboratories around the world. The techniques demonstrated to date introduce modal discrimination simply by increasing the optical loss for the higher-order modes or by simply increasing the relative gain of the fundamental transverse optical-mode.
Generally, in the case of VCSEL diode lasers, the laser beam obtained from these semiconductor devices usually comprises several different transverse modes. This leads to a divergence of the laser beam emitted by such solid-state semiconductor diode lasers. In order to obtain a high quality and less divergent beam, the problem of eliminating or reducing the level or the intensity of the transverse modes occurring within the solid-state semiconductor diode laser""s cavity, consequently arises.
In addition, the pumping efficiency of a solid-state semiconductor diode laser""s cavity is low, particularly in the case of a so-called stable cavity (e.g., planoconcave) solid-state semiconductor diode laser. There is also the problem of lowering the operating threshold of the previously described solid-state semiconductor diode lasers (i.e., the incident power necessary for operating the solid-state semiconductor diode laser). Prior art also describes that the pumping of solid-state semiconductor diode lasers is no longer coveted by conventional side emitting laser diodes, but now also exists within the realm of the xe2x80x9cVertical Cavity Surface Emitting Laserxe2x80x9d (VCSEL) as well. As already stated VCSELs are semiconductor, lasers having vertical cavities that comprise electrically pumped multiple quantum well materials that also act as the VCSEL""s optically pumped laser active medium.
However, the thickness of the VCSEL""s active laser medium is very small; this is because the VCSEL""s active laser medium only comprises a few quantum wells. Typically, the previously mentioned laser active medium that comprises a few quantum wells is surrounded by two light reflecting mirror structures, which especially in the case of VCSEL devices, are normally constituted by a successive epitaxial deposition of semiconductor thin films. The typical axis of a VCSEL""s vertical cavity is perpendicular to the VCSEL""s multilayered structure, hence the term xe2x80x9cvertical cavityxe2x80x9d laser.
Typically, a laser beam is emitted from a VCSEL""s surface area, hence the term xe2x80x9csurface emittingxe2x80x9d lasers, as opposed to side emitting semiconductor lasers, which emit their laser beams from the laser""s edge containing a partial light reflecting prism facet into a direction parallel to an edge emitting semiconductor laser""s multilayered structure. VCSELs as laser pumping mechanisms, however, suffer from certain disadvantages. Firstly, the power density emitted by a VCSEL is low, being typically only about xe2x80x9c50xe2x80x9d milliwatts for a xe2x80x9c30xe2x80x9d millimeter beam; whereas, a conventional side emitting solid-state semiconductor diode laser might emit, for a same surface area, a power of approximately xe2x80x9c1xe2x80x9d Watt.
However, despite the disadvantages a laser beam emitted by a VCSEL diode laser has good properties. A VCSEL pumped solid-state semiconductor diode laser has a much better beam quality than a solid-state semiconductor diode laser pumped only by a conventional diode. The reason for this is because the structure typical to VCSELs (i.e., a very short cavity length) causes a natural reduction in beam divergence, while lowering the operational threshold of the solid-state semiconductor diode laser it pumps, as well. The spectral width in the emission of a VCSEL is lower than that of a conventional diode but comparable to the spectral width of the absorption band of solid-state microlasers (.DELTA..lambda..sub.abs .apprxeq.1 nm for a YAG microlaser, .DELTA..lambda..sub.emission .apprxeq.0.3 nm for a VCSEL and .apprxeq.3 nm for a 1 W power pumping diode).
Moreover, the geometry of the laser beam typically emitted by present day VCSEL devices is circular, symmetrical, not rectangular, and does not suffer from anamorphosis, as is the case with side emitting solid-state semiconductor diode lasers. This circular geometry permit an easier coverage of the laser beam emitted by a VCSEL device and the circular pattern typically exhibited by light occurring at the VCSEL""s fundamental mode.
More particularly, in the case of a stabilized solid-state semiconductor diode laser cavity, the shape of the laser beam emitted by a VCSEL device is well adapted to the pumping of the present invention""s fundamental mode. VCSELs emit a lower power density than conventional side emitting solid-state semiconductor diode lasers, but the previously mentioned density is better distributed for pumping light at the fundamental mode. Hence, there is a better pumping efficiency and, consequently a better pumping of solid-state semiconductor diode laser""s fundamental mode.
In addition, the use of a solid-state laser active medium in addition to the active semiconductor material used to comprise a laser""s light emitting diode active-region typically constitutes a layer of limited material thickness (i.e., between xe2x80x9c150xe2x80x9d and xe2x80x9c1000xe2x80x9d microns) and limited diameter size (i.e., several square millimeters). If, for design purposes, a solid-state active-medium material needs to be located within the optical cavity of its optical pumping semiconductor diode laser, the material used in constructing the solid-state semiconductor diode laser""s active-medium needs to be epitaxially deposited onto the top and outermost surface of the last quarterwave plate used to construct the solid-state semiconductor diode laser""s first quarterwave mirror-stack assembly, i.e. sometimes called the bottom or base xe2x80x9cDistributed Bragg Reflectorxe2x80x9d (DBR). If, for design purposes, a solid-state active-medium material needs to be located outside the optical cavity of its optical pumping semiconductor diode laser, the material used in constructing the solid-state semiconductor diode laser""s active-medium needs to be epitaxially deposited onto the top and outermost surface of the last quarterwave plate used to construct the solid-state semiconductor diode laser""s second quarterwave mirror-stack assembly, i.e. sometimes called the top xe2x80x9cDistributed Bragg Reflectorxe2x80x9d (DBR). Depending upon the design parameters of the laser device in question, the previously mentioned solid-state active-medium could be optically pumped from within a solid-state semiconductor diode laser""s optical cavity, or it could be optically pumped from outside the solid-state semiconductor diode laser""s optical cavity. Where the material that comprises a solid-state active-medium is either directly hybridized onto a solid-state semiconductor diode laser itself (i.e., inside or outside the solid-state semiconductor diode laser""s resonant optical cavity), or the material that comprises a solid-state active-medium is coupled to the latter using an optical fiber.
In addition, the combination of a VCSEL diode laser and solid-state active-medium (i.e., YAG:Nd) offers several advantages over some prior art, including being easy to produce using any well known process of epitaxial deposition in the construction of its successive layers, all of which have different natures and functions (e.g., mirror, active medium, saturable absorber, etc.). The manufacturing a solid-state VCSEL diode lasers would require substrates of considerable dimensions (i.e., diameter xe2x80x9c1xe2x80x9d to xe2x80x9c2xe2x80x9d inches, i.e. xe2x80x9c25.4xe2x80x9d to xe2x80x9c50.8xe2x80x9d mm), where from the previously mentioned substrate it will be possible to produce several hundred or possibly several thousands solid-state VCSEL diode laser devices. Manufacturing processes well known within the microelectronics industry are now used to cut and assembly solid-state VCSEL diode lasers and, therefore makes it possible to collectively stage the isolation and assembly of each chip into a diode laser device.
In summarizing, prior art shows how the structure of a VCSEL diode laser makes it possible to improve the quality of the laser beam emitted by a solid-state semiconductor diode laser, especially, by improving the geometrical characteristics of the beam emitted by the solid-state VCSEL diode laser, by increasing the differential efficiency of the solid-state VCSEL diode lasers, and by reducing the solid-state VCSEL diode laser""s operating threshold (i.e., in incident power).
Furthermore, to better understand the structural differences between the present invention""s FCSSL design and prior art solid-state VCSEL diode laser technology, a typical example of a solid-state VCSEL diode laser design is described in detail below. Prior art, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 describes a solid-state VCSEL diode laser device, which uses a well known process of recombination xe2x80x9celectron/holexe2x80x9d radiation (i.e., what is sometimes called xe2x80x9cradiative recombinationxe2x80x9d) to produce within an optical cavity electrically pumped fundamental light. The main difference between this prior art solid-state VCSEL diode laser design and most other prior art solid-state VCSEL diode laser designs is this prior art solid-state VCSEL diode laser design electrically pumps its diode active-region directly using two contact-layers, while most prior art solid-state VCSEL diode laser designs electrically pump their diode active-regions using an electrically conductive substrate and electrically conductive quarterwave mirror-stack assemblies, while they optical pump their solid-state active-medium""s externally, by epitaxially depositing the solid-state active material outside their vertical cavities.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows a high-frequency version of a solid-state VCSEL diode laser design that uses its metallic supporting substrate 22 (FIGS. 1, 2, and 3) as both a base-reflecting mirror 22 and as a material growing substrate that is used for the subsequent growth of the various multilayered structures that will ultimately comprise the high-frequency solid-state VCSEL diode laser""s physical structure. Multilayered material growth that begins with at a substrate material layer is typically the deposition process VCSEL devices use in epitaxially growing the contiguous layers that will ultimately comprise their multilayered structures. This is process is usually accomplished using a well-known material deposition process like MBE or MOCVD to epitaxially deposit the material in question. Hereinafter, the term xe2x80x9cVCSEL(s)xe2x80x9d, instead of the previously used term xe2x80x9chigh-frequency solid-state VCSEL diode laser(s)xe2x80x9d will be used to represent prior art.
Furthermore, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 a VCSEL""s material supporting substrate 22, when made conductive as an alternative embodiment serves as the VCSEL(s) electrically negative electrode. The metallic supporting substrate 22 is comprised from a (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d alloy-mixture, which has between an xe2x80x9c8.0xe2x80x9d to a xe2x80x9c12.0xe2x80x9d percent material lattice-mismatch, or more specifically, a xe2x80x9c10.0xe2x80x9d percent material lattice-mismatch to the binary (GaN) xe2x80x9cGallium-Nitridexe2x80x9d semiconductor material deposited later. Despite the (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d lattice-mismatch exhibited by the material, it is still the preferred metallic alloy-mixture used for this kind of electron conducting metallic supporting substrate 22. In addition, the (Nixe2x80x94Al) xe2x80x9cNickel-Aluminumxe2x80x9d metallic supporting substrate 22 (FIG. 3), if used as an alternative embodiment, would also need to exhibit a highly reflective property as well and, therefore should have a surface roughness of less than xe2x80x9c15xe2x80x9d atoms thick.
Furthermore, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows several thin layers of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d material, which are grown layer-upon-layer using MBE or MOCVD as the deposition process, until a buffer-layer 23 (FIG. 3) is built up epitaxially that has a thickness of only a few atoms. This buffer-layer 23 is used for facilitating the MBE or MOCVD epitaxial growth of the many subsequent semiconductor layers that will entirely comprise the high-frequency VCSEL device.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that typically a high-frequency short-wavelength VCSEL design would have a lower xe2x80x9cDistributed Bragg Reflectorxe2x80x9d (DBR) or quarterwave mirror-stack assembly 24 grown, epitaxially, onto the top and outermost surface of the previously mentioned buffer-layer 23A, 23B, 23C, 23D (FIGS. 1, 2, and 3) of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d material using any suitable epitaxial crystal growing method, such as MBE or MOCVD. Moreover, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that a high-frequency VCSEL""s quarterwave mirror-stack assembly 24 is typically constructed as a plurality of mirror pairs; or more precisely, as comprising a multitude of single pairs of alternating layers 24A, 24B (FIGS. 1, 2, and 3), which are constructed using xe2x80x9cGallium-Nitridexe2x80x9d and xe2x80x9cAluminum-Gallium-Nitridexe2x80x9d (GaN/AlGaN) semiconductor materials. The plurality of mirror pairs will include one or more layers of N-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d 24A, 24C, 24E, 24G, 241 (FIGS. 1, 2, and 3), which is a high refractive semiconductor material used to construct the first layer within each mirror pair, and N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d 24B, 24D, 24F, 24H, 24J (FIGS. 1, 2, and 3), which is a low refractive semiconductor material used to construct the second layer within each mirror pair.
For example, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that a layer 24A of N-doped (GaN) xe2x80x9cGallium-Nitride is epitaxially deposited onto the top and outermost surface of a VCSEL""s last buffer-layer 23D (FIGS. 1, 2, and 3), while a layer 24B (FIGS. 1, 2, and 3) of N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d is subsequently and epitaxially deposited onto the top and outermost surface of the VCSEL""s first N-doped (GaN) xe2x80x9cGallium-Nitride layer 24A, which form a VCSEL""s first single mirror pair. If additional mirror-pairs are required, several more layers are used to make-up additional mirror-pairs, which are deposited, epitaxially, onto the existing layers of (GaN) xe2x80x9cGallium-Nitride and (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d materials 24A, 24B, 24C, 24D, 24E, 24F, 24H, 24I (FIGS. 1, 2, and 3).
Moreover, to increase the reflectivity of a VCSEL""s quarterwave mirror-stack assembly 24 (FIGS. 1, 2, and 3) to any required amount of reflectance, many additional mirror pairs may be required, and depending on the frequency of light being reflected, as many as several hundred pairs might be needed and used. However, it should be understood that the thickness and doping levels of each deposited layer used within VCSEL devices must be precisely controlled. Any deviation from designed parameters, no matter how slight, would affect the performance of the VCSEL device (i.e., frequency range, flux intensity). This greatly adds to the cost and complexity of manufacturing high frequency VCSEL devices and really VCSEL devices in general.
For example, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that a VCSEL""s emitter-layer 34 (FIGS. 1, 2, and 3), being designed to emit high-frequency laser light having a wavelength of xe2x80x9c200xe2x80x9d nanometers, should have a material thickness that is one-quarter of one wavelength of the optical radiation to be emitted by the VCSEL device in question.
Consequently, to insure the amplification of a VCSEL""s intra-cavity produced fundamental light and the extraction of frequency specific lased light this quantity must be the same for all of the layered material used to construct the VCSEL""s first quarterwave mirror-stack assembly 24.
Therefore, prior art, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that all of the layers used in the construction of a VCSEL""s first quarterwave mirror-stack assembly 24 (FIGS. 1, 2, and 3) have a material thickness of xe2x80x9c50xe2x80x9d nanometers. The doping of specific layers within a VCSEL device is accomplished by the addition of various dopant materials (e.g., n-type electron donating dopants like Phosphorus and p-type electron accepting dopants like Boron). Material doping of specific layers is normally accomplished using a epitaxial deposition process like MBE or MOCVD, during which vaporized dopant material or gaseous dopant material is injected into a plasma mixture containing the deposition material(s) needing to be doped, where addition of a dopant material to a particular semiconductor material occurs during that particular semiconductor material""s layered epitaxial deposition.
Typically, a VCSEL device will use many different dopant concentrations of specific dopant materials within the several different extrinsic semiconductor layers that make-up a VCSEL""s multilayered structure. For example, alternating layers of (GaN) xe2x80x9cGallium-Nitridexe2x80x9d 23A (FIGS. 1, 2, and 3) and N-doped (AlGaN) xe2x80x9cAluminum Gallium Nitridexe2x80x9d 23B (FIGS. 1, 2, and 3), which are used to facilitate construction of a VCSEL""s first quarterwave mirror-stack assembly 24 (FIGS. 1, 2, and 3) can be made n-type conductive when doped with either xe2x80x9cSeleniumxe2x80x9d or xe2x80x9cSiliconxe2x80x9d using a dopant concentration ranging from xe2x80x9c1E15xe2x80x9d to xe2x80x9c1E20xe2x80x9d cubic-centimeters with a preferred range from xe2x80x9c1E17xe2x80x9d to xe2x80x9c1E19xe2x80x9d cubic centimeters, while a nominal concentration range of doping would be from xe2x80x9c5E17xe2x80x9d to xe2x80x9c5E18xe2x80x9d cubic centimeters. The percentage of dopant composition used in a VCSEL""s first quarterwave mirror-stack assembly 24 could be stated as (Alxc3x97Gaxc3x97N/GaN), where x represents a variable of xe2x80x9c0.05xe2x80x9d to xe2x80x9c0.96xe2x80x9d, while in a preferred embodiment x would represent an amount greater than xe2x80x9c0.8xe2x80x9d.
Moreover, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that the plurality of alternating layers used to construct a VCSEL""s first quarterwave mirror-stack assembly 24 are typically deposited onto the top and outermost surface of the VCSEL""s buffer-layer of (AlN) xe2x80x9cAluminum-Nitridexe2x80x9d 23 using a well known process of epitaxial deposition.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows the layer next in line for epitaxial deposition is a VCSEL""s first contact-layer 25 (FIGS. 1, 2, and 3), which is comprised from a highly +n-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d binary semiconductor material epitaxially grown onto the top and outermost surface of the last alternated layer used in the construction of the VCSEL""s first quarterwave mirror-stack assembly 24 (FIGS. 1, 2, and 3). A VCSEL""s first contact-layer 25, while providing connectivity to the VCSEL""s n-metal contact 27 (FIGS. 1, 2, and 3), and to the VCSEL""s n-metal contact-ring 26 (FIGS. 1, 2, and 3), also enhances the reliability of the VCSEL""s design by preventing the migration of carrier-dislocations, and the like, to the VCSEL""s active-region 28 (FIGS. 1, 2, and 3).
Furthermore, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 also shows that to prevent the overcrowding of the cladding-layers within a VCSEL""s active-region 28, each cladding-layer is illustrated as being a single separate layer 28A, 28C (FIGS. 1, 2, and 3). It should also be understood that each cladding-layer could also be made using more than one layer, where each cladding-layer 28A, 28C would be epitaxially deposited onto a previous cladding-layer, where each cladding-layer 28A, 28C would be comprised from a N-doped or a P-doped (AlGaN) xe2x80x9cAluminum-Gallium-Nitridexe2x80x9d ternary semiconductor material, or any other frequency specific and application determining doped material available.
Furthermore, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that the next layer(s) next in line for epitaxial deposition is a VCSEL""s active-region 28 (FIGS. 1, 2, and 3), which is shown as being represented by either a single layer presented here as comprising a (SQW) xe2x80x9cSingle Quantum Wellxe2x80x9d epitaxially deposited onto the top and outermost surface of the VCSEL""s first cladding-layer 28A (i.e., sometimes called a cladding-barrier). It should be understood, however, that a VCSEL""s active-region 28 could also include one or more quantum-wells and one or more quantum-well cladding-layers, as is typical of MQW structures. More particularly, a first quantum-well cladding-layer, a second quantum-well cladding-layer, and a quantum-well layer 28B shown as being positioned between a first cladding-layer 28A and a second cladding-layer 28C. Prior art, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 describes a VCSEL""s active-area 28B as being comprised as a SQW that is constructed from a p-doped (InGaN) xe2x80x9cIndium-Gallium-Nitridexe2x80x9d extrinsic ternary semiconductor material using MBE or MOCVD to epitaxially deposit the material onto the top and outermost surface of the VCSEL""s first cladding-layer 28A.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows the next layer in line for epitaxial deposition is a VCSEL""s second contact-layer 29 (FIGS. 1, 2, and 3), which is comprised from a highly +p-doped (GaN) xe2x80x9cGallium-Nitridexe2x80x9d extrinsic binary material that is epitaxially grown onto the top and outermost surface of the VCSEL""s second cladding-layer 28C. A VCSEL""s second contact-layer 29, while providing connectivity to the VCSEL""s p-metal contact 31 (FIGS. 1, 2, and 3), and to the VCSEL""s p-metal contact-ring 30 (FIGS. 1, 2, and 3), will also enhance the reliability of the VCSEL""s design by preventing the migration of carrier-dislocations, and the like, to the VCSEL""s active-region 28.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows the next layer in line for epitaxial deposition is a VCSEL""s solid-state laser-active medium 32, which typically constitutes a layer of limited material thickness (i.e., between xe2x80x9c1xe2x80x9d and xe2x80x9c1000xe2x80x9d microns) and limited diameter size (i.e., one to several microns in diameter) that is epitaxially deposited onto the top and outermost surface of the VCSEL""s second contact-layer 29. The material used to comprise the laser-active medium layer 32 is typically chosen from one of the following materials: YAG (Y.sub.3 Al.sub.5 O.sub.12), LMA (LaMgAl.sub.11 O.sub.19), YVO.sub.4, YSO (Y.sub.2 SiO.sub.5), YLF (YLiF.sub.4), or GdVO.sub.4, etc. Choice criteria for one or other of these materials are given in EP-653 824 (U.S. Pat. No. 5,495,494). This document also gives information on the choice of the thickness of the active-medium, particularly for obtaining a monomode laser, typically, the thickness of the active-medium is approximately: xe2x80x9c750xe2x80x9d micrometers for a YAG active-medium, xe2x80x9c500xe2x80x9d micrometers for an YVO.sub.4 active-medium, xe2x80x9c150xe2x80x9d micrometers for a LMA active-medium. Concerning the doping ions, a choice is generally made of neodymium (Nd) for a laser emission around xe2x80x9c1.06xe2x80x9d micrometers. It is also possible to choose Erbium (Er) or an Erbium-Ytterbium codoping (Er+Yb) for an emission of around xe2x80x9c1.5xe2x80x9d micrometers. For an emission around xe2x80x9c2xe2x80x9d micrometers a choice is made of Thulium (Tm), or Holmium (Ho), or a Thulium-Holmium codoping. Doping with Ytterbium makes it possible to obtain an emission at xe2x80x9c1.03xe2x80x9d micrometers. It is also possible to produce an active-medium constituted by glass, e.g. doped with Erbium and Ytterbium (i.e., emission at xe2x80x9c1.55xe2x80x9d micrometers), as explained in the article by P. Thony et al. entitled xe2x80x9c1.55 micrometers wavelength CW microchip laserxe2x80x9d, Proceedings of Advanced Solid-State Laser 1996, San Francisco, where the doped glass active-medium is consequently dielectric.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows the layer(s) next in line for epitaxial deposition is a VCSEL""s second quarterwave mirror-stack assembly 33, which is made from a plurality of structures comprised as mirror pairs. More precisely, a plurality of mirror pairs comprising two epitaxially deposited layers 32A, 32B (FIG. 3) are constructed when two optical materials having opposed refractive indices are alternately used during a mirror pair""s deposition.
For example, prior art as presented here, describes a single mirror pair as comprising two epitaxially deposited layers, which are constructed using xe2x80x9cAluminum-Oxidexe2x80x9d a high-refractive optical material deposited as the first layer in each mirror pair, and xe2x80x9cZinc-Oxidexe2x80x9d a low-refractive optical material deposited as the second layer in each mirror pair, these materials having opposed refractive indices create a highly reflective mirror pair of (Al2O3/ZnO). Ion sputtering, a well-known method for depositing dielectric materials is used to create all of the mirror pairs of (Al2O3/ZnO) that will ultimately comprise a VCSEL""s second quarterwave mirror-stack assembly. The construction of additional mirror pairs is accomplished by a repeated deposition of additional layers, which are alternatively constructed using xe2x80x9cAluminum-Oxidexe2x80x9d and xe2x80x9cZinc-Oxidexe2x80x9d materials, alternating the deposition of xe2x80x9cAluminum-Oxidexe2x80x9d material with xe2x80x9cZinc-Oxidexe2x80x9d material, next alternating back to xe2x80x9cAluminum-Oxidexe2x80x9d material. To explain further, mirror pairs are the building blocks used to construct a VCSEL""s second quarterwave mirror-stack assembly, which will include one or more layers of undoped (Al2O3) xe2x80x9cAluminum-Oxidexe2x80x9d a high-refractive dielectric material (i.e., sometimes called Corundum or manufactured Sapphire) 33A, 33C, 33E, 33G, 331 (FIG. 3), and one or more layers of undoped (ZnO) xe2x80x9cZinc-Oxidexe2x80x9d a low-refractive dielectric material 33B, 33D, 33F, 33H (FIG. 3). For example, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 describes a VCSEL""s second quarterwave mirror-stack assembly as comprising first a xe2x80x9c200xe2x80x9d nanometer thick layer of (Al2O3) xe2x80x9cAluminum-Oxidexe2x80x9d 33A, which is epitaxially deposited onto the top and outermost surface of a VCSEL""s laser-active medium layer 32 (FIG. 3). Next, a first xe2x80x9c200xe2x80x9d nanometer thick layer of (ZnO) xe2x80x9cZinc-Oxidexe2x80x9d 33B (FIG. 3) is subsequently and epitaxially deposited onto the top and outermost surface of the previously deposited first xe2x80x9c200xe2x80x9d nanometers thick layer of (Al2O3) xe2x80x9cAluminum-Oxidexe2x80x9d 33A (FIG. 3) material, thereby forming a first mirror pair of (Al2O3/ZnO). If additional mirror-pairs are required, than several more layers, used to comprise additional mirror-pairs, can be deposited onto the top and outermost surface of the existing layers of alternatively deposited (Al2O3) xe2x80x9cAluminum Oxidexe2x80x9d and (ZnO) xe2x80x9cZinc Oxidexe2x80x9d materials 33A, 33B, 33C, 33D, 33E, 33F, 33G, 33H, 331 (FIG. 3). To increase the reflectivity of a VCSEL""s second quarterwave mirror-stack assembly 33 (FIG. 3) to the required amount of partial-reflectance, many additional mirror pairs will be required, and depending upon the frequency of light being reflected, as many as several hundred mirror pairs might be used in creating a VCSEL""s second quarterwave mirror-stack assembly.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows a VCSEL""s emitter layer 34 as being constructed from undoped (ZnO) xe2x80x9cZinc-Oxidexe2x80x9d, a highly refractive dielectric amorphous optical material used to create the VCSEL""s last deposited layer, which is epitaxially deposited onto the top and outermost surface of the last layer of xe2x80x9cAluminum-Oxidexe2x80x9d that was used to construct a VCSEL""s second quarterwave mirror-stack assembly. Deposition is accomplished by using a well-known method for depositing dielectric materials called ion sputtering, which is normally used to epitaxially deposit dielectric amorphous materials like (ZnO) xe2x80x9cZinc-Oxidexe2x80x9d onto an existing layer.
In addition, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that a VCSEL""s p-metal contact 31 and p-metal contact-ring 30 are formed onto the top and outermost surface of the VCSEL""s second contact-layer 29 (FIGS. 1, 2, and 3), by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys. Prior art, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 describes how a VCSEL""s n-metal contact 27 (FIGS. 1, 2, and 3) and n-metal contact-ring 26 (FIGS. 1, 2, and 3) are formed onto the top and outermost surface of the VCSEL""s first contact-layer 25 (FIGS. 1, 2, and 3), which is typically accomplished by disposing any suitable conductive material, such as Indium-Tin-Oxide, Gold, Zinc, Platinum, Tungsten, or Germanium metallic alloys. Furthermore, it should also be understood that a chosen method of material deposition depends upon which material is selected for constructing a VCSEL""s electrical contacts 26, 27, 30, 31 (FIGS. 1, 2, and 3). Therefore, specific methods of material disposition, disposing, and patterning onto the VCSEL""s first and second contact-layers 25, 29, for any specific material, must be considered in the construction of the VCSEL""s electrical contacts 26, 27, 30, 31 (FIGS. 1, 2, and 3).
Moreover, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows that the next layer in line for deposition is a VCSEL""s second contact-layer 29 (FIGS. 1, 2, and 3), a VCSEL""s second cladding-region 28C, a VCSEL""s active-area 28B, and a VCSEL""s first cladding-layer 28A (FIGS. 1, 2, and 3), all of which are mesa-etched structures, which will define the overall shape and structure of the VCSEL""s lower layers, while diameter dimensions for a VCSEL""s lower layers remain substantially larger over the VCSEL""s top deposited emitter-layer 34 (FIGS. 1, 2, and 3) and the emitter-layer""s support 29. As mesa etching of a VCSEL""s main structures are completed a VCSEL""s p-metal contact 31 (FIGS. 1, 2, and 3) and p-metal contact-ring 30 (FIGS. 1, 2, and 3) are to be deposited onto the top and outermost surface of the VCSEL""s second contact-layer 29 leaving, therein the VCSEL""s emitter-layer area open 34.
In addition, the deposition of a VCSEL""s n-metal contact, as an alternative embodiment, can be deposited onto the top and outermost surface of the VCSEL""s metallic supporting substrate 22 (FIGS. 1, 2, and 3) comprised from (NiAl) xe2x80x9cNickel-Aluminumxe2x80x9d alloy, which would allow the metallic supporting substrate 22 to also function as an electrically negative contact-layer. Prior art, as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows a VCSEL""s metallic supporting substrate-layer 22, when it is used in conjunction with a quarterwave mirror-stack assembly 24 (FIGS. 1, 2, and 3) that is constructed as a multitude of mirror pairs using highly reflective (AlGaN/GaN) xe2x80x9cAluminum-Gallium-Nitride/Gallium-Nitridexe2x80x9d material, will provide for approximately xe2x80x9c99.99xe2x80x9d percent of the VCSEL""s total reflectivity. Furthermore, prior art as illustrated in FIGS. 1, 2, 3, 4, 5, and 6 shows how high-frequency VCSEL devices can be grouped together and configured as a linear array of high-frequency surface emitting diode lasers.
However, while the present invention might typically use a vertically oriented double-heterostructure light emitting diode to produce fundamental photonic radiation, its folded cavity, and the physics that occur there, is quit different from those that occur within a typical solid-state VCSEL diode laser. Therefore, the present invention should by all accounts be categorized as a new type of solid-state semiconductor diode laser.
For example, the present invention uses only one mirror structure to provide for both a highly reflective input mirror and a partially reflective output mirror, while replacing the normally used second mirror altogether with a polyhedral shaped prism waveguide, which is constructed from a transparent solid-state laser-active medium material like YAG, YVO, or LMA. Obviously, the present invention has a novel and redefined vertical cavity that will provide preferentially gain for the fundamental lower-order optical-mode, which will result in single transverse lower-order optical-mode output emissions. Inspired by the present invention""s unique folded cavity design, the fact that its polyhedral shaped prism waveguide is constructed from a solid-state active-medium, and because of the optical physics that occur therein, I have for future identification named this new solid-state semiconductor diode laser design the xe2x80x9cFolded Cavity Solid-State Laserxe2x80x9d (FCSSL).
In accordance with the present invention, a Folded Cavity Solid-State Laser comprises a cavity folding polyhedral shaped prism waveguide having at least one internal reflecting prism that provides a cavity folding transverse redirection of intra-cavity produced fundamental photonic radiation back into the light-amplifying laser-active material that was used in its construction (i.e., YAG:Nd, YVO, or LMA), a semiconductor double-heterojunction light emitting diode active-region comprising an active-area, which provides both the electrical production and the optical pre-amplification of intra-cavity produced fundamental photonic radiation, a dichroic-mirror structure capable of reflecting a sufficient amount of undiffused optical radiation, while providing the feedback apparatus that redirects intra-cavity produced optical radiation radiation back into the laser-active material (i.e., YAG:Nd, YVO, or LMA) used to construct the polyhedral shaped prism waveguide for further amplification, while providing an apparatus that will also provide a frequency-selected output of frequency-specific monochromic amplified photonic radiation.
Objects and Advantages
Accordingly, besides the objects and advantages of the Folded Cavity Solid-State Laser described in the above patent, several objects, and advantages of the present invention are:
(a) To provide a folded cavity solid-state laser that creates a high output of narrow-linewidth amplified light using a cavity folding internal reflecting polyhedral prism waveguide comprised from a single layer of optically amplifying laser-active material;
(b) To provide a folded cavity solid-state laser that is inexpensive to manufacture, because it eliminated the expensive epitaxial deposition of a bottom positioned quarterwave mirror-stack assembly comprising a multitude of layers, and replaced it with a single dielectric polyhedral prism waveguide, which is constructed from a single inexpensive layer of sputter or epitaxially deposited material;
(c) To provide a folded cavity solid-state laser that uses two graded confinement cladding-layers to generate higher output emission;
(d) To provide a folded cavity solid-state laser that produces a more effective output gain using two graded confinement cladding-layers to lower the heat which is produced by electrical resistance between current conducting contact-layers and cladding-layers;
(e) To provide a folded cavity solid-state laser, which increases optical confinement with the addition of total internal reflecting cladding material to the surrounding vertical and outermost wall surfaces of the folded cavity solid-state laser""s folded vertical cavity(s);
(f) To provide a folded cavity solid-state laser, which can be configured and controlled as a single folded cavity solid-state laser device;
(g) To provide a folded cavity solid-state laser, which can be configured as a single folded cavity solid-state laser-array comprising a multitude of diodes, which are controlled as a single group of folded cavity solid-state lasers (i.e., sometimes called a solid-state laser-array) or controlled as a single group of independently controlled folded cavity solid-state lasers;
(h) To provide a folded cavity solid-state laser or a folded cavity solid-state laser-array, which can be manufactured at the same time and from the same semiconductor substrate material used to construct the laser-array""s control-circuitry, all of which, would be contained within a single integrated semiconductor device;
(j) To provide a folded cavity solid-state laser that replaces a bottom quarterwave mirror-stack assembly with a polyhedral prism waveguide which, if comprised of quartz or fused silica, can totally reflect one-hundred percent all frequencies of optical radiation that enters a polyhedral prism waveguide""s top front-face flat horizontal surface;
(k) To provide a folded cavity solid-state laser, which can inexpensively construct its polyhedral prism waveguide using a well-known ion-milling process to slice out the waveguide""s prism facet(s);
(l) To provide a folded cavity solid-state laser that can deposit a dielectric material like fused-silica onto any construction material that might be used to construct any frequency producing semiconductor diode or combination thereof that could possibly be used to construct a folded cavity solid-state laser or a folded cavity solid-state laser-array;
(m) To provide a folded cavity solid-state laser, which uses an amorphous material like xe2x80x9cLithium-Fluoridexe2x80x9d (LiF) to create an optical cladding layer, which is deposited around each vertical cavity or cavities, creating a structure providing for the total internal reflection of intra-cavity produced light, thermal dispersive, and added support to a folded cavity solid-state laser""s polyhedral prism waveguide structure(s);
(n) To provide a folded cavity solid-state laser, which increases its output, while maintaining a narrow line-width for its output emissions(s);
(o) To provide a folded cavity solid-state laser, which has eliminated the need to pre-deposit buffer layers of growth promoting materials like xe2x80x9cAluminum-Nitridexe2x80x9d onto a diode(s) substrate layer;
(p) To provide a folded cavity solid-state laser, which will produce an increase of nearly 7-mW to its output emissions;
(q) To provide a folded cavity solid-state laser, which increases gain to its emission output by using a laser active medium like YAG:Nd as the material used in the construction the folded cavity solid-state laser""s polyhedral prism waveguide.
Further objects and advantages are to provide a folded cavity solid-state laser, where the selection of a semiconductor or the selection of an optical material over another that might be used in the construction of the folded cavity solid-state laser""s active-region, polyhedral prism waveguide, and quarterwave mirror-stack assembly is not determined by any structural need or lattice compatibility, but by a particular application""s need for frequency specific laser output. The materials used in the construction of the present invention FCSSL as presented here are only one yet preferred example of a group of several frequency-specific materials that might be used in the construction of the present invention""s frequency-transcendent multilayered structure. The advantage is that the novel features and the un-obvious properties that lie behind a folded cavity solid-state laser""s cavity folding structure, because they can exist, and occur, using any frequency-specific semiconductor or optical material, shows that the various structures that comprise folded cavity solid-state laser(s) have a sufficient novelty and a non-obviousness that are independent of any one particular semiconductor or optical material that might or could be used in the construction of folded cavity solid-state laser(s).
Still further objects and advantages will become apparent from a consideration of the ensuing description and drawings.